texts: Could Black holes forge heavier elements that have yet to be discovered?

texts:
Observations

The heaviest elements known in abundance in nature are forged deep within stars.
These elements are made possible by the high densities/pressures within the stars.
Black holes are known to have a much higher density/pressure than any known star.
Black holes are also known to be a phase of stellar evolution - this suggests that the original star's internal process of forging metals would persist within the resultant black hole.
Scientists have forged synthetic/ephemeral heavy metals under conditions which could hypothetically be sustained within a black hole.

Hypothesis:

Black holes forge heavier elements that have not been observed on earth. The conditions needed to sustain these elements are unique to the black hole, due to its high density/pressures. These conditions can be glimpsed, but not sustained in any experimental context

Follow up questions:

Has this been hypothesized?
Where can I find research on this topic?


texts:
I've read a little bit about the universe, but its not clear to me if there is a place in the universe beyond which there are no stars.

For example, is there anywhere in the universe that one could go where we would not generally see stars in every direction?  Its seems uncertain to me that there is.

If I were to go to the farthest star from earth magically, rather the via relatively (for argument sake).  I"m guessing it would have stars surrounding it in every direction.  But perhaps I'm wrong, perhaps there is some edge to the explosion of matter from the big bang, beyond which there is nothing.

Do we know if the universe curves in on itself? or is there some other dominant theory?


texts: Would you expect this planet to be hotter or colder than earth?

texts:
I have a lab where given the mass of a star being $1.23$ times the math of our sun, some planets wobble of central start is $273$ days, and the velocity of 79 m/s. I found that the mass of the planet is $5.3\times 10^{27}$ kg and is approximately $.82$ AU away from its star. Here is a screen shot of my excel file:

Now I am asked would we expect this planet to be hotter or colder than earth?

I believe this question is rather silly since we have no idea the composition of the planet or other related factors to make any conclusion. From what I found is based on the mass and radius it is probably a water world, and since it is pretty close to its star I would say that it is probably hotter than earth.

Anyways, any suggestions or thoughts on this matter would be appreciated.


texts: Where would be the absolute most logical place in USA to observe the total solar eclipse on August 21, 2017?

texts: Cloudy weather would defeat the purpose so I am guessing that the western states would be more ideal due to greater chance of clear sky.  Isolation would also be nice.  At a camp site on or near a mountain top.  Also, are there efforts to set up mobile solar observatories in the totality path?


texts: Gift for 16 year old Aspiring astrophysicist,,,,

texts:
I'm an outsider here, so please bear with me.
I have a 16-year-old nephew interested in the field of astrophysics and am looking for a birthday gift.  I had thought about a book on astronomy, but don't see that field listed as a core pre-requisite for that career.  Is knowledge of astronomy a core pre-requisite/useful for this profession?
Thank you


texts: How does diffraction occur in a reflecting telescope?

texts:
It's understandable how diffraction occurs in refracting telescopes, because the lens itself is a small aperture for light to pass through.

But what about reflecting telescopes? Since light is reflected, and a mirror is not exactly an 'aperture'? How can light diffract in a reflecting telescope?


texts: In astronomy class, we have to state how gravity work in the black hole? I know for one fact is that black hole is strong enough to stretch you thin enough like a noodle. Any ideas?


texts: Mass accumulation rate and orbit-time of earth

texts:
According to me following reason of mass accumulation of earth has been completely overlooked so far.
The sunrays that fall on the oceans travel deep into water and finally get converted into mass which eventually results in "mass-accretion of the earth". So earth is continuously getting heavier due to the mass being added to it due to this reason year by year.
Therefore, should this also not be one of the reasons why the orbit-time of the earth (time taken by it to take one round of the sun) is bound to change (increase or decrease) with time?


texts: Options for first time astrophotography w/ 80/900 refractor?

texts:
Recently I bought a telescope and I started to take pictures of my observations. The most basic solution, I'm using a smartphone adapter and my Xiaomi Mi 6 camera to do that but the images (except for the Moon sometimes) is too bad and Jupiter and its moons not even show in my screen. So I thought in use a more serious device to take the pictures.

I have in my personal finance control a monthly quota to "astronomy", but to start I think in spend about 200€ (~$230,00).

I work with programming for 10 years (C#), so I can use something more "crude" if needed (like Raspberry Pi or Arduino). You guys can suggest something to start take decent shots of  Jupiter and its moons?


texts: Looking for a routine to correct for Local Standard of Rest (LSR)

texts:
I've been scanning the Milky Way with a Small Single Dish Radio Telescope, obtaining Spectral data with a Span of ~ 2 MHz, centered at the 21 cm line. With this information I can derive a relative velocity (due to Doppler shifted emission) between the observer (telescope, at lat ~ 33° South) and gas clouds in the line of sight. To study the position of this clouds relative to the Galactic Center, I know that the way to correct for this is subtracting our motion relative to this 'fixed' point: correcting by Earth's rotation, Earth motion on the Solar System plane and the System's motion itself (everything projected onto the line of sight), so my data is comparable to any other observation.
I've been reading on related questions such as

The Solar Motion and the peculiar velocities of stars

What are galactic speeds measured against?

How spectrographs that measure radial velocities manage to translate variations in the stars' spectrum lines into the “speed” of the star?

I also looked on "Tools of Radio Astronomy", T.L Wilson, K. Rohlfs, S. Hüttemeister, 5ed., pg. 189. that briefly describes this procedure.

The thing is I can't find any routine explained in detail, about this:

So I wonder how this is really done in practice and if there's any, e.g., Python Package that does it (maybe in Astropy?).
EDIT: Thanks to @ELNJ 's response and help I managed to get agreeable results. This is data from a Standard Region, particularly S9.
def Freq_to_v(freqval, freq0, mjd_time = 59244.8, obs_ra = 268.1, obs_dec = -34.43):

    #https://docs.astropy.org/en/stable/api/astropy.coordinates.SkyCoord.html#astropy.coordinates.SkyCoord.radial_velocity_correction

    #This is the observation for S9, 30-01-2021, 16:34 hrs CL.
    #S9 Std. Reg. Obs. time = 59244.8

    t = Time(mjd_time, format='mjd', scale = 'utc')

    #Obs. Location and height in GEODETIC coordinates (and not geocentric a X Y Z quantities tuple!!).

    loc = EarthLocation(lon= 289.47, lat =-33.27, height = 1450)

    sc = SkyCoord(1*u.deg, 2*u.deg)

    vcorr = sc.radial_velocity_correction(kind='barycentric', obstime=t, location=loc).to('km/s')
    #####################################################

    #Regular Doppler Shift Calculation. Here are two conventions: optical and radio doppler shift.

    shift_opt, shift_rad = (freqval - freq0)/freqval, (freq0-freqval)/freq0 #shifting factor, no dimensions.

    v_opt, v_rad = const.c.to('km/s')* shift_opt , const.c.to('km/s') * shift_rad #velocities.

    #################################################### To barycentric:

    rv = v_rad - vcorr #corrected to barycentric w/o Special Relativity terms.

    #This is the observation for S9 Std, Region., 30-01-2021, 16:34 hrs CL.

    ################################################### To LSR:

    my_observation = ICRS(ra=obs_ra *u.deg, dec= obs_dec *u.deg, \
     pm_ra_cosdec=0*u.mas/u.yr, pm_dec=0*u.mas/u.yr, \
     radial_velocity= rv, distance = 1*u.pc)

    new_rv = my_observation.transform_to(LSR()).radial_velocity

    return new_rv

You can compare Williams' 1972 result

with mine:

and check that it's in fact consistent.


texts: In what ways does the mass and energy of the milky way curve space time?

texts: Space is curved by mass (or more precisely by mass and momentum as described by the stress-energy tensor). The milky way system has a lot of mass and a lot of energy. In what ways is space-time curved within it?


texts: How did the Archimedes crater fill with lava?

texts:
This is the Archimedes crater, on the moon, near the Apollo 15 landing site.

It is located in Mare Imbrium, a plain filled with lava.
The strange thing is that the interior of the crater looks flat, as if it had been also filled with lava, but there is no passage for lava to come inside the crater. All the walls appear to be intact.
So how could lava have possibly entered the crater?
Could it have had such low viscosity that it infiltrated as seepage?
The interior of the walls appears to be broken as if they suffered a landslide.
(The image source is this video, apparently a Lunar Reconnaissance Orbiter image)


texts: What galaxy looks the most like the Milky Way?

texts: Is there a particular galaxy cataloged by astronomers which is considered to most closely resemble what the structure of the Milky Way is believed to look like, when viewed from a similar distance?


texts: How to compensate the effect of tectonic activity in devices like LIGO?

texts:
In late 2015, the LIGO project team announced the detection of gravitational waves. The detector is (very basically) a laser measuring the distance between two mirrors in vacuum, over a large-enough distance to detect very small changes.

The two mirrors are installed on Earth, and a couple kilometers apart.

I believe, perhaps mistakenly, that tectonic activities (if only vibrations) may change the mirror distance over time, so the LIGO project has probably used some model and technology to nullify their effects.

How can that be done? If there is any mistake there, I wonder about how to validate the experiment results.


texts: Can a solar system exist where the second planet rotates fast, and the third planet is tidally locked to their star?

texts:
Today I read about the Romulan home star system and it looks like it might be impossible for such a star system to exist.

Memory Alpha describes Remus:

  Remus was tidally locked, with one hemisphere always facing its primary while the other always faced away from it. The civilization of the Remans was located on the night side.

Remus is farther from their star than Romulus. Romulus rotates many times during a single orbit of their star while Remus is tidally locked and rotates once per orbit of their star.

If a planet orbits even deeper in the gravity well of their star than Remus it should be even more likely to be tidally locked than plants further out.

Could tidal interactions with neighboring planets and/or its moons prevent a planet from becoming tidally locked while being unable to prevent planets further out from being tidally locked?


texts:
I tried posting this on math stack exchange to no avail so I thought maybe it's more relevant here :
I know a similar question has been posted here before but no links to any mathematical derivations given on the following post. I was not able to access the citations on the wiki page either.
How do I calculate the sun's azimuth based on zenith, hour angle, declination, and latitude?
I was recently trying to derive the position of the Sun in the sky given the latitude, time of year and time of day.
I used the following definitions:
$\phi :$ I used this to denote the angle of latitude, as per convention
$t:$ I used this to denote the angle of the year completed since Summer solstice
$\theta:$ I used this to denote the angle of the day completed since solar noon
$\Omega:$ I used this to denote the solar zenith angle.
$\Gamma:$ I used this to denote the azimuth of the Sun clockwise from due North
$k:$ I used this to denote the angle of 66.56 degrees between the Earth's axis and plane of revolution
$\lambda:$  I used this to denote the angle between the Earth's axis and the vector in the direction of the sun, this is complementary to the more commonly used declination angle $\delta$
My goal was to get a function $f(\phi, t, k,\theta)=\Omega,\Gamma$
I was able (with some help) to derive the following formulae:
$\cos(\lambda)= \cos(t)*\cos(k)$
$\cos(\Omega)=\cos(\lambda)*\sin(\phi)+\cos(\phi)*\sin(\lambda)*\cos(\theta)$
However, I was not able to then derive the formula given on the following Wikipedia page for $\Gamma=f(\theta,\lambda,\Omega)$ which I have transcribed into my variables below:
https://en.wikipedia.org/wiki/Solar_azimuth_angle
$\sin(\Gamma)=-\dfrac{(\sin(\theta)*\sin(\lambda))}{\sin(\Omega)}$
I think in order to derive this I need to convert to a local co-ordinate system with due North along the y-axis, due East along the x-axis and zenith along the z-axis.
Can someone help me to proceed?


texts: Can someone in a deep well see the stars in daytime?

texts:
I am wondering whether it is possible for a guy in a deep well to see the stars in the daytime.

Now the background is dark. So might the stars be visible?


texts: When was the term "orbit" (in the modern sense) first used and by whom?

texts: Wikipedia coyly suggests that some notions and views kept circulating (apparently since Adam and Eve) until finally Kepler, of all people,  ... etc. I find this hard to believe. Please elucidate.


texts: Where can I find raw text, ascii, or binary data from the GALEX satellite?

texts:
I am looking for stars in the GALEX data through the GALEX data using GALEXview and plugging in their Henry Draper Numbers

http://galex.stsci.edu/galexview/

But the task is tedious by hand. Is there any place that I can look to find the raw text, ascii, or binary data? I want to be able to simply pull the data I need in one go with a program, rather than copying it down by hand. Currently I'm having to look through the data point star by star.

http://galex.stsci.edu/GR6/

Thanks!


texts:
I've recently been introduced to relativistic celestial Reference Systems (RS) such as the Geocentric Celestial Reference System (GCRS) and Barycentric Celestial Reference Systems (BCRS). So, for example we use GCRS to model near-Earth satellites or use BCRS to model probes far into our solar system. I understand the need for such RS i.e. to be in compliance with the theory of relativity - no favoured coordinate systems, limitation of the speed of propagation of information etc.

However, what are some layman definitions of advantages of such systems to really send the message home?

I notice that when describing $n$-body dynamics in our solar system for example, Barycentric coordinate time is used instead of proper time. This blows my mind!


texts: Equivalence of minor epicycle and eccentric

texts:
In epicycle-deferent astronomy, adding a second ”minor” epicycle to account for observational discrepancies is observationally equivalent to shifting the deferent into a so-called eccentric, or a circle with a center not at the Earth (see e.g. DeWitt 2010, Worldviews, p. 120, and the illustrations I’ve borrowed from that chapter), as was pointed out already by Hipparchus. Ptolemy famously chose the eccentric approach for the Almagest. I’m trying to wrap my head around how to geometrically prove the equivalence.

More precisely, the idea I am after is that the addition of a second epicycle to the original epicycloid (compounded only of a deferent and a major epicycle) is supposed to have the same effect upon the movement as projected unto the outer circle of fixed stars by a line of sight from Earth as just shifting the original epicycloid by a distance equal to the diameter of the minor epicycle. I think it’s the earliest proof of an ”empirical equivalence” of two hypotheses.


texts: How does interstellar matter density vary?

texts:
Answer to my question partially answers this one, about density of intergalactic matter and matter within galaxy:

  But it is mostly a hot, ionized void. How void? The density of the intergalactic medium is about 1 to 100 particles per cubic meter (you can compare it to the mean galactic density, of about a million particles per cubic meter, or that of Earth's atmosphere, of about 10^26 particles cubic meter). How hot? It can go from 10^5 to 10^7 K.

If we skip the most dense concentrations of matter (stars, planets, generally everything solid, liquid or plasma, and border conditions like their atmosphere) how dense interstellar matter can we find? What is the concentration of matter in densest nebulae that still don't collapse into bodies like planets or stars?

And conversely, how empty does the space get at its emptiest? I could imagine only very few particles in last their short moments of travel into the center exist under a black hole's event horizon, but other than that, how empty a space can be found in the universe, and where?


texts: Why do some celestial bodies have atmospheres, and not others?

texts: Question is rather self-explanatory. An answer touching upon how/why atmospheres are formed in the first place would be ideal as well. An example of such astral bodies would be The Earth and it's moon. Earth has an atmosphere, while the moon does not. Why is that so?


texts: How to obtain distance modulus and Total extinction from Isochrone fitting to CMD?

texts:
This is my first post here.

I am studying the following article: https://arxiv.org/abs/1901.03574.

On Page 5, Sec 3.1, the authors are presenting their results they obtained via Isochrone fitting to the CMD of the two GCs being discussed. My questions are:

How can I find out the distance modulus and the Total extinction $A_{F606W}$ from just the fitting? I know that Isochrone fitting is done when we need to estimate the age. In order to fit, we need to move the Isochrone both vertically (Distance modulus?) and horizontally (Color excess/Reddening/Extinction?). Is my understanding correct?
How do they convert the Reddening in filters to reddening in B-V? What is the technical details/physics of it?
What is the role of the Extinction law in getting these numbers?
In the end, they calculate the distance, which is simple once we get the distance modulus. Why is it the distance from the Sun? Did we assume a heliocentric coordinate system somewhere?

Thank You.


texts: Why does the background noise in this image of 2020 QG look like corduroy?

texts:
The news item Small asteroid becomes closest ever seen passing Earth: NASA contains a "handout image" with the caption:

This NASA/JPL/ZTF/Caltech Optical Observatories handout image obtained on August 18, 2020 shows asteroid 2020 QG (the circled streak in the center), which came closer to Earth than any other nonimpacting asteroid on record NASA/JPL-CALTECH/AFP

Question: I saw what looks like a corduroy-like pattern in the background that is likely from some processing artifact. Does anybody recognize this kind of artifact? Does it look familiar?

above: NASA image from linked article. below: Corduroy source.

Fourier analysis (log power) of the image of the asteroid, the two spots demonstrating the strong linear periodic modulation of the background.
Concentric rings were seen when analyzing only the red part of the initial annotated image. Now that I average all three colors they are almost gone. The inclined line in the FT is probably that of the bright, straight linear trail of the asteroid.

import numpy as np
import matplotlib.pyplot as plt

fname = '424852de3808b4d6df1101d3b4f0790afd3b9d13.png'

img = plt.imread(fname)[..., :3].sum(axis=2) # sum 3 colors makes red circle gray to minimize impact on FFT

print(img.shape)

f = np.fft.fftshift(np.fft.fft2(img))
p = np.abs(f)**2

plt.figure()
c0, c1 = [int(s*2**-1) for s in p.shape]
hw0, hw1 = [int(s*2**-3) for s in p.shape]
plt.imshow(np.log10(p[c0-hw0:c0+hw0, c1-hw1:c1+hw1]), vmin=5, vmax=8)
plt.gca().set_aspect(16/9)
plt.colorbar()
plt.title('log10(ft power)')
plt.show()


texts: What are some applicable problems with the correct usage of G?

texts:
In some fields of astrophysics, where distances are measured in parsecs (pc), velocities in kilometers per second (km/s) and masses in solar units (), it is useful to express G as approximately 4.302 x 10^-3 pc M^-1 (km/s)^2 ...
Is G not approximately 6.67 x 10^-11 N(m/kg)^2? Any reasonable explanation would be appreciated.


texts: How to calculate the altitude of the Celestial Equator from an observer's latitude? Exam question

texts:
When observing from a latitude of 50 degrees North on Earth, the Celestial Equator will have a maximum altitude of:
  Answer: 40 degrees

I've looked this up and it says something about the observer's co-latitude. However, I do not understand why this is the case. Could someone explain this please?


texts: Can New Horizons probe turn back and start orbiting Pluto

texts: New Horizons probe has found astonishing features on Pluto, while the likelihood of making even greater discoveries is remote in further space. Is it possible for New Horizons probe to turn back and start orbiting Pluto? It may save much time and resources as compared to sending another probe to Pluto. Thanks for your responses.


texts: Can a black hole formation be stopped or interrupted

texts: My question is a hypothetical: if I were to create a black hole the size of a family house, in New York, is there any way that it can be stopped, or interrupted? If a black hole of this size is formed, will everyone die?


texts: Population of excited H levels in a Strömgren Sphere

texts:
In chapter 2.2 of Astrophysics of Gaseous Nebulae and AGN Ostriker and Ferland claim that, as far as ionization is concerned, one can assume all atoms to be in the ground state in a Strömgren Sphere setting (static, homogeneous $n\approx 10/\mathrm{cm}^3$, isothermal $T\approx 10^4\,\mathrm{K}$ pure hydrogen cloud around a single star). While I do not doubt the validity of this assumption, I have some trouble with how they justify it.

They estimate the inverse ionization rate for ground state hydrogen at a typical distance from the star, which is of course the lifetime against ionization from the ground state $\tau_\mathrm{ion}^{1^2S}$. Then they say that the estimate also holds for ionizations from excited levels and they find

$$\tau_\mathrm{ion}\gg\tau$$

where $\tau_\mathrm{ion}$ and $\tau$ are the typical lifetime against excitation and the typical lifetime of excited states respectively.
So far so good. In the next step, however, they end their chain of argumentation by saying that consequently any atom in an excited state will have enough time to decay to the ground state before it can be ionized, so all ionizations are from ground state. That is my understanding at least.

My problem with this is the following: suppose I have some mechanism that keeps a good fraction of my atoms in an excited state. In that case, even if $\tau_\mathrm{ion}\gg\tau$, I will have ionizations from this excited level simply because it is always populated. So what one should really look at in my opinion is the lifetime of an atom against excitation.

My first question is: Am I missing something or did I get something wrong?

And secondly: If my argumentation makes sense, one has to compare the lifetime against excitation with $\tau$, right? Where can I find the corresponding cross sections for radiative and collisional excitation? I was only able to find ionization cross sections.


texts: Some face-on spiral galaxies show barred structure in their central bulges, whereas some edge-on spiral galaxies have boxy bulges. I wonder if there is any relation between these two. In particular, do boxy bulges indicate barred spirals?


texts: Can you identify this planet seen in the night sky?

texts:
I saw a very bright star in the night sky on Monday (Jan 30, 2017). The star was in the Western sky and was directly above the crescent moon.

My location is Toronto, Ontario. Canada. It was about 8pm at night, but by 10pm the star had disappeared over the horizon.

I'm sure this was a planet as it shined brighter then any of the other stars, but I couldn't guess which one.

Can you identify this planet for me?


texts: When did the Scutum constellation receive its current name?

texts:
While cleaning my desk, I found an out of date star map (puts the cleaning into perspective, huh?) where the Scutum constellation is still referred to as Scutum Sobiescianum).  This is the original name Johannes Hevelius gave it in 1684

My map is (unfortunately) modern and comes from the 2nd half of the 20th century. It's a Polish one so it's not odd for it to still use the traditional name. The main illustration of the Scutum article in Polish Wikipedia happens to do so as well.

I know the name of the constellation has been changed but I can't find any information on who did it and when. I suppose it must have been the IAU (International Astronomical Union) but I have no idea regarding the time of the change.

When was the English name of the constellation officially changed and who did it?


texts:
So recently it was announced that there might be a ninth planet out there.

Some people (from NASA) say the odds are 50% that it is a real planet, since it hasn't been discovered by telescopes yet.

Let's say there is no ninth planet. Is there another explanation for those orbits that require a big mass (8 times earth mass) out there?

Could it even be a very small black hole (a few centimeters in radius) or maybe a clump of dark matter?


texts: How does the Earth move in the sky as seen from the Moon?

texts:
I just want to be sure I am visualizing this correctly, because it seems odd. The Moon is tidally locked to the Earth but there are wobbles to its motion due to libration. So from a point on the surface of the near side of the Moon, the Earth would always be near the same place in the sky? It would describe a small circle or a side-to-side wobble over the course of a month, but never move far from that point?

That would seem very strange, like it was a gigantic stage prop or something. We are so conditioned that everything rises and sets (except for a few stars near the poles).


texts: How do focal length, angular magnification and field of view relate?

texts:
The larger the focal length the larger the angular magnification The smaller the focal length the larger the field of view.

I don't understand why this applies. I'm talking specifically about telescopes.

Wouldn't a greater angular magnification imply a greater field of view too?


texts: Why don't we orbit the center of our galaxy?

texts: At the center of the milky way, there must be some giant mass that keeps the galaxy from just floating away from itself. So, obviously, we feel the immense gravitational pull of whatever this is. But, on a smaller scale we can see this with the earth and the sun. The sun keeps us from spinning into oblivion with its gravitational pull. The question I am asking is, why do we orbit the sun, but not the center of the galaxy, which has a much stronger gravitational pull?


texts: Could the merger of two rotating black holes create another that breaks the rotational speed limit?

texts:
The black hole GRS 1915+105 was observed to be spinning at near the theoretical maximum spin rate.

Assuming you had two such black holes spinning clockwise, and also orbiting each other clockwise, what would happen if they merged? How could the new black hole avoid breaking the rotational speed limit? Where would the extra angular momentum go?


texts: Will the first Event Horizon Telescope image of the Milky Way's black hole be just another orange donut?

texts:
@HDE226868's answer to Why doesn't the black hole in the center of the Milky Way glow similarly to the famous M87 image? explains that neither black hole is orange and any Event Horizon Telescope images will be made at about 1 mm (~300 GHz). It goes on to point out that there's going to be far less emission from Sgr A*'s accretion disk than there is from that one. Of course ours is a lot closer, but it's not always active; unless a dust cloud wanders by and falls in. For more on that see answers to Why not take a picture of a closer black hole?
@RobJeffries' answer to How can the Event Horizon Telescope image Sgr A when it's not visible from all sites at one time?* points out that our disk might also vary in appearance over timescales of minutes rather than months, so it's going to be a new challenge in terms of data processing.
Question: When all is said and done though, will the first EHT image of Sgr A* look pretty much like M87; a lumpy orange donut? I know there is an issue of inclination, but it won't look like it's edge-on because of the way light bends around, so I think it's necessary to consult a simulation. Another issue is the accretion source itself; will there be a disk or just spiraling filaments? Assuming the same wavelength and baseline there is a possibility of higher resolution, and the shorter timescale compared to M87 might be offset by a much sorter distance and therefore brightness.


texts: orbital angular momentum and the longitude of ascending node

texts: Consider the motion of a planet around an oblate star. Because of the oblateness the star has a non-zero quadrupole moment which causes the orbital angular momentum of the planet to precess around the symmetry axis of the oblate star. My naive question is: is this precession rate exactly the same as the precession rate of the longitude of the ascending node or are there subtle differences between the two?


texts: Explaining gravity assist to a 6-year old

texts:
We've been talking about Cassini with my 6-year old daughter recently and I've mentioned that it picked up more speed by passing close to Venus (twice), Earth and Jupiter. But then, I've struggled to explain how did it pick up speed, why and how does it work.

How would you recommend to explain/demonstrate the concept of "gravity assist" to a 6-year old?


texts: Can the range of the observable universe be extended through an intermediary?

texts:
I understand that the so called observable universe has a limit we cannot see through.

My question is whether we might get information through an intermediate hypothetical actor located halfway to the limit. This actor would have, I assume, a limit of the observable universe further away than ours.

We, in turn, might interact with this actor in order to get information about a nova we cannot see from the Earth.

Information about such a star might perhaps have reached the Earth earlier at a time before it changed into a nova. The information about the nova would therefore be new.


texts: Thinking of buying a new telescope. What aspects should be considered?

texts:
I'm new to astronomy and I have a Celestron 50Z but I'm thinking of buying a better telescope.
I see that there are many options out there but I'm not really sure as to which aspects I should consider that make a good telescope.

Can anyone point me to the right direction? If you ask how much I'm willing to pay, I'd say up to US $300.

Any suggestions not just on telescopes but as I said, things I should be considering which make a good telescope worth buying.


texts: Why do astronomers use the equatorial coordinate system that moves, instead of one based on fixed, distant stars?

texts:
As an outsider to the field, I am surprised to learn that astronomers commonly use the equatorial coordinate system, in which fixed stars are not actually fixed.

Isn't that making life harder than it needs to be? Why would one not define a coordinate system in terms of fixed stars?

For example like this: Fixed star A is defined as (0°, 0°), fixed star B is defined as (0°, X). That would constrain the 3 degrees of freedom of a spherical coordinate system.

Just to be clear, by fixed stars I mean stars that do not change their apparent relative position over long time scales, say a million years. So stars in the Milky Way would likely be excluded.

When making a new observation, these two reference stars may not be visible, which would make calibration difficult. But there could be a reference catalog of fixed stars with precisely known coordinates. Any new observation could be made relative to a visible subset of that catalog. This would be similar in principle to the International Temperature Scale of 1990, which defines temperature relative to a set of reference materials and states.


texts: Apart from Io, will any Galilean moons have oceans and land if their ice melts

texts: When our sun expands earthlings, if still alive, will have to leave. The Galilean moons offer a lot of water but we would also need land.  As far as I can tell there will be no land for earthlings to live on when the oceans of these moons melt. Is anyone aware of land  masses on these moons and if so how large?


texts: How many times has the Milky Way rotated?

texts:
I would think that is a highly naive question, but I still ask it.

Given current experimentally discovered numbers:

the Milky Way galaxy is ~13-14 billion years old
At the galactic radius where our sun is, the rotational period is about 240 million years.

So, with nothing changing at all (the galaxy and stars pop fully formed into existence, the rotation period doesn't change), the galaxy has rotated roughly

  ~14/(1/4) = 50 to 60 times.

Since the assumptions are naive, what is a better approximation for the number of times? How long has the mass that is recognizable now as the Milky Way been rotating (and what has been the speed by radius over time)? Is it a lot more than 50 times and if so, has it been slowing down as a whole or just at the Sun's position, or a less because the coalescence into stars rotating took a few billion years to translate into the faster rotation which it is now mostly set at? Or some other reason which is more articulate?

Note: This question is similar to but distinct from The equation for how many times the Earth has revolved around the galaxy, as the title asks similarly what I'm asking but the content just asks for how to calculate rotational speed.


texts: Has J been used by itself as a spectral type?

texts:
While answering the question "Why have brown dwarf classes been dubbed L, T and Y?" I noticed that in the paper Kirkpatrick et al. (1999) "Dwarfs Cooler than 'M': The Definition of Spectral Type 'L' Using Discoveries from the 2 Micron All-Sky Survey (2MASS)", the letter "J" was ruled out as being in use as a standard carbon star type.

So far I have not seen a source that uses "J" as a carbon star type by itself, I've only seen it in combination with "C", e.g. "C3,3J" or "C-J4 C2 5" (both from XHIP). This contrasts with the other carbon star types "R" and "N" which have definitely been used as separate identifiers.

Has "J" ever been used as a stand-alone spectral type, e.g. "J3"?


texts:
why doesn't a solar eclipse occur in every new moon?

I know one reason is the inclination of Earth's orbital plane. But is there another?


texts:
When calculating local sidereal time, 100.461 "is the value needed to yield the correct value of GMST" - David Hammen (left as comment on my previous question). How is this value calculated?

If (hypothetically) I wanted to use January 1st 12:00 (noon) 2010 as my epoch, how would I calculate this number?


texts: Telescope showing high chromatic aberration

texts: Hi i have made a telescope with 1100 mm focal length objective lense and 21mm plano convex as eyepiece. Both these lenses are not achromatic. But when is looked through it , the images were completely blurry. Is this because I have not used achromatic lense.?? Can it be resolved if I used any coated eyepiece.? The diameter of objective is 7.5 cm and eyepiece is 1cm. Is this issue is because the size of eyepiece is so small.


texts: How bright is it on Pluto in the middle of the day?

texts:
I just saw a documentary about New Horizons and Pluto with plenty of photos New Horizon had taken of Pluto. All these photos were very bright, like Earth would be during the day. Considering the distance between Pluto and the sun I suspect the brightness of the photos has been manipulated.
So, if you would be standing on Pluto's equator when the sun is in zenit, how bright would it be?


texts: How to convert horizontal coordinates using NOVAS?

texts:
I'm using NOVAS 3.1. I know that I can convert equatorial coordinates to horizontal coordinates using the equ2hor function.

Is it possible to make NOVAS do the inverse transformation: from horizontal to equatorial? There is no hor2equ, but maybe some other function implements this functionality?


texts: What error bar / confidence level is generally reported in galaxy distances?

texts: In reporting distances of galaxies, what are the typical error bars and associated confidence levels?


texts:
I was thinking about the possibility of determining what phase of evolution a variable star is in when it is located in the instability strip (say, a Cepheid object). Can you differentiate whether it is evolving toward the blue (in HR diagram) or back toward the giant branch?

I was looking at a figure recently in Carroll and Ostlie's Intro to Astrophysics text, which brought this to mind. It shows on the HR diagram where Cepheids lie, which is right in this strip.


texts:
I can look at a star chart and identify things like constellations in the sky.  But if I take a picture with my DSLR camera (35mm with a decent zoom lens, no astronomy specific optics), I run into difficulty. I find that the camera can see a lot more stars than my eye, and I find that I start to have great difficulty even identifying simply constellations.  For instance, when I look for the constellation Cassiopeia in the night sky it is very easy to spot: it looks like a sideways "W".  But when I take a picture I don't see a W at all in the picture, I see about 16 stars around where the "W" should be a weird angles to each other, and it's hard to figure out which stars to connect to form the actual constellation.

And it just goes downhill from there, as other constellations don't even have a shape as easily recognizable as Cassiopeia, and as a result I have a great deal of difficult even recognizing what I am looking at.

A photograph with a regular camera is not quite the same as looking through a telescope, because the camera has a much wider field of view.  And at the same time, there does not seem to be a linear relationship between the actual magnitude of the star and how bright it looks.  As I mentioned above, I see 16 stars which look somewhat comparable in brightness, but I know that they differ by several magnitudes.

Is there some kind of trick or technique that I can use to quickly identify stars in pictures I take?  In looking for clues, I have not seen anybody mentioning this problem.


texts: Are sulfur and phosphorus also created in the nucleosynthesis of big stars?

texts:
There are as far as I know two  fusion reactions by which stars convert hydrogen to helium: The CNO cycle (for carbon–nitrogen–oxygen) and the  proton–proton chain reaction.
The elements created in the CNO cycle are C,N,O and Fluor.
But in this video min:13:00 https://www.youtube.com/watch?v=pxrPaNDvaQE the prof. talks about Phosphorus and sulfur. Is that true and are those element also created in the CNO cycle or is he talking about something else?


texts: Centerless expansion or a matter of perspective

texts:
I get this question a lot:

  Where is the center of the Universe?

I see this question on the internet all of the time:

  Where is the center of the Universe?

Now I have a question regarding all of the answers that I always either give or read:
The question linked is the last question and answer that I saw before posing this question here, and is in no way the same question, as this question asks how the concept of a centerless universe can work, whereas the question below asks about how a universe can be perceived to be centerless even when it might not be.

Is the Universe centerless? Or does it simply appear to be because of our perspective?

$\\\\$
This is about to be a bad example, but please bear with me.

If you were a bird floating in the ocean, among a myriad of other birds, and you let time pass such that every bird floated away from the other birds in some random fashion. Would you, at some later time when everybirdy was roughly separated from one another, say that based upon your observations of the present conditions, and some evidence that at one point all of the birds were together, that the ocean is centerless? Or could you assume that because of your position in the flock, that from your perspective you can't tell where the center is, nor do you think that from a different birds position would you be able to tell? Creating the illusion of a centerless ocean.

Obviously this requires you not be at or close to the very edge of the flock.

Edit: Also see examples in KenG's posed answer and comments.


texts: How long do thunderstorms on Venus last?

texts:
A while ago, I learned about field of exometeorology (the study of other planets ' atmospheres) which fascinates me. In particular, I read about thunderstorms on Venus, e.g. about the results from ESA's Venus Express Mission in 2006, or according to Universetoday.com: Lightning Storms on Venus Similar to Those On Earth:

“We have analyzed 3.5 Earth-years of Venus lightning data using the low-altitude Venus Express data, which is about 10 minutes per day,” Russell said. “By comparing the electromagnetic waves produced at the two planets, we found stronger magnetic signals on Venus, but when converted to energy flux we found very similar lightening strength,” said Russell. Also it seems that lightning is more prevalent on the dayside than at night, and happens more often at low Venusian latitudes where the solar input to the atmosphere is strongest.”

This is mainly a statement about the intensity and time distribution of the lightning on Venus. I am now wondering whether there is more known about the mesocale convective systems on Venus. My search gave me the impression that there is not much literature on that topic (yet), or am I mistaken?
The main question I am after: How long do storm cells on Venus last? Is any mean or median known? Can thunderstorm systems live for multiple (Venus) days?
References
Although I did not find any answer to my question, here some references I found:

C. C. Covey & G. Schubert: Mesoscale convection in the clouds of Venus, Nature 290, pages 17–20 (1981)
M. Nakamura, T. Imamura, et. al. Overview of Venus orbiter, Akatsuki, Earth, Planets and Space volume 63, pages 443–457 (2011)
List of all missions to Venus according to Wikipedia
NASA's In-Depth Page about Venus mentions the clouds and their lightnings briefly.
Akatsuki (あかつき, 暁, "Dawn") as Venus-orbiter, is currently (as of 2020) the only existing artifical Venus satellite, I suppose.


texts: How does the evolution of a solar system not break the second law of thermodynamics?

texts:
Please forgive: I am a layman when it comes to physics and cosmology, and have tried finding an answer to this that I can understand, with no luck.

As I understand it, the solar system evolved from a massive molecular cloud. To me, this seems to break the second law of thermodynamics, as I think it suggests order from disorder.

I know there must be something wrong with my logic, but am really stuck.

Can anyone explain this one in layman's terms?

(Posting to both "Astronomy" and "Physics", as it seems to overlap these subjects)


texts: How to calculate apparent brightness in relation to the sun

texts:
I know that we can determine the apparent brightness of an object in our solar system using the following formula:
$$B=\frac{A\cdot L_S\cdot R^2}{D^2\cdot d^2}$$
$B$ = Brightness of observed object in watts
$A$ = Albedo of observed object
$L_S$ = Luminosity of the Sun in watts
$R$ = Radius of the observed object in meters
$D$ = Distance between the Sun and the observed object in meters
$d$ = Distance between the Earth and the observed object in meters
However, I find this isn't helpful in visualizing the brightness in your mind. I was wondering what modifications would be needed to this formula to make the result relative to the sun.


texts: My professor used the term 'local star'. What range of distance is generally considered 'local' for a star? And what would be considered distant?


texts: Does Hawking radiation in fact bring mass into the universe?

texts:
As I understand virtual particles pairs, e.g. electron and positron spontaneously pop into existence for tiny period of time, then they annihilate.

When virtual electron and virtual positron annihilate we should see two photons emitted to opposite directions, right? If so, shouldn't we detect photons appearing from vacuum for every such pair?

Also, when a pair of virtual particles pops near event horizon of a black hole one particle may be captured by the black hole, while the other escapes it. Both particles have mass and energy. Thus caught particle brings the black hole its mass. Then how the black hole loses mass through Hawking radiation? Shouldn't it gain mass instead?


texts: Is a black hole heavier than the star from which it was created?

texts:
I did some googling around and couldn't get answer to a simple question of black hole mass compared to the star from which it was created. It is well known that giant stars burn very bright and relatively very quickly. They end up their lives as red giants and eventually collapse into a black hole.

Suppose that the star is mid-life and has some mass M. Over the time it burns its fuel, turns into a red giant and after supernova, turns into a black hole. This black hole has some mass B. Why B >> M?

According to some simple physics laws, mass could not be created nor destroyed.


texts: How do I find stars within milky way using galactic latitude and longitude

texts: I have data that contains galactic latitude and longitude of stars. I have to determine all the stars that are within our milky way. How do I find it?


texts: What is a singularity? What is at the center of a black hole? Specifically regarding space-time

texts:
So because I can only really think of space-time in 2-dimensions like a sheet of something, my assumptions might be wrong to begin with. I was watching a YouTube video on black-holes and there was a quip regarding black holes and their warping of space time, to the point where the narrator said something along the lines of “there is nothing at the center of the black hole but gravity has warped space time so much that it's this ’pinhole’ like affect that causes the blackhole behavior”.

My questions are:

When a star dies and collapses into a black hole, what is at its center? The star's mass compacted into the size of the plank length, or something similarly small? Is there really nothing at the center of a black hole? Surely, the core collapsed into something really small right?
What is meant by singularity? Is it just the warping of space-time that makes it this way?


texts: Stratospheric Balloon with an Xray camera

texts: Some colleagues and I were planning to launch a stratospheric balloon in December. A thought occurred to me: what if we were to put some sort of xray camera in the payload? We are expecting a maximum altitude of about 80,000 feet. Would it work any better at that altitude?


texts:
I am trying to get some data from the Sloan digital sky survey (SDSS). This task turned out to be challenging for me as someone with only little knowledge about astronomy and maybe someone here can help me. What I need is data for all galaxies of the SDSS with their optical spectroscopy and position.

I suppose I need:

redshifts
RA (right ascension)
dec (declination)
classifications of all SDSS spectra

from the Optical Spectra Catalog Data

I found a script to convert ra/dec/redshift to x/y/z coordinates.
Will I need "the associated photometric position based matches" and "stellar parameter (SSPP) results" to get RA and dec? I have no idea what these are.


texts: Assuming you could be standing inside a black hole, would it be light or dark around you?


texts:
The Phys.org news item Hubble is the ultimate multitasker: Discovering asteroids while it's doing other observations includes the image below.

What causes these asteroid trails to be so strangely shaped and repeated? Usually asteroids appear as short straight segments in long exposures from terrestrial telescopes.

  Some asteroids from within our Solar System have photobombed deep images of the Universe taken by the NASA/ESA Hubble Space Telescope. The asteroidal streaks in this image are created by our virtual neighbours; asteroids in the main asteroid belt between Mars and Jupiter. In the background are thousands of colourful galaxies, some of them billions of light years away. Credit: NASA, ESA, and B. Sunnquist and J. Mack (STScI); CC BY 4.0; Acknowledgment: NASA, ESA, and J. Lotz (STScI) and the HFF Team


texts: Is there anything like a timelapse showing the Alpha Centauri stars orbiting around each other?

texts:
It is my understanding that the angular separation between Alpha Centauri A & B is more than enough for even amateur telescopes to resolve as 2 separate stars. Going by the plot below, over the past 20 years there should be noticeable movement showing them orbiting. Do we have any images/videos that capture this movement?

Source click for full size


texts:
A nearby star has crossed Chandrasekhar's limit. How do we come to know when it will undergo supernova explosion?
Or has it already undergone supernova explosion?


texts: Can IR sensitive cameras read signals from Venus's surface emissions?

texts:
This question got me thinking about this.

I know that we measure the Earth's surface temperature by satellite (perhaps somewhat inaccurately, but it's done all the same).   Using Venus as an example, can we see see evidence of Venus' 800 degree surface from space or is Venus' temperature deduced more by modeling and the Russian ship that landed there.

All things with heat give off a thermal signature and that signature isn't a single wavelength but a range of wavelength.   I assume, by measuring the overall color emitted and/or the peak wavelength, it's a fairly straight forward calculation to work out the temperature.

But when you have an atmosphere, and this is the main part of of my question.   I assume, an atmosphere is to some degree transparent, so you'd be seeing in a sense, heat through a medium and inside as well as on the upper surface.  But the heat you see diminishes depending on how opaque/transparent that atmosphere is to the specific wavelength.

Are there any good ballpark estimates to how thick (or how dense or pressurized), we can see temperature signatures through an atmosphere?     In the case of Venus, does any of it's 800 degree surface temperature generated IR make it through it's atmosphere, where it would be observable by spacecraft, giving a good measurement of it's surface temperature, or was it's surface temperature deduced by different types of models and that mistreated Soviet spacecraft that didn't last long once it had landed.

I imagine some atmospheric gases are more transparent than others.

Just to clarify - a specific "yes/no we can detect IR from Venus' surface" and a more general answer as to how far we can see heat through an atmosphere, are both fine.


texts: Plotting Histograms of stellar mass and gas mass given a halo mass

texts:
I am an undergrad doing a first course in astronomy. I am working on a small project. In it I have been assigned a halo mass (1.444e12 $M_{sun}$). I want to plot histograms of possible stellar masses and gas masses for 100 galaxies having mass close to the given halo mass.
Now I have studied the stellar-halo mass relation and figured out that the possible stellar mass for given halo mass can range between 1e10 to 1e11 $M_{sun}$. But I don't know the exact distribution of stellar masses in this range.
It would be helpful if anyone could give me a headstart on how I could do this?


texts:
In a paper titled

  Arp, H. (1984). Distribution of quasars on the sky. Journal of Astrophysics and Astronomy,
  5(1):31-41.

at Journal of Astrophysics and Astronomy, Halton Arp wrote that there is a notable clustering of quasars at $21^h<RA<3^h$, and a marked lack of quasars at $9^h<RA<15^h$, but I haven't been able to find other references to the same distribution. The paper was published in 1984, so has anyone else published a similar result, or has this observation been

refuted, or
explained?


texts: What is period doubling in a variable star?

texts:
I'm coming across the concept of "period doubling" in  RR Lyrae stars.  When I look up what period doubling is on the Internet, I find general-level explanations of bifurcations of dynamical systems and transitioning to chaos. However, I am unable to apply these things to stars in a way I understand.

What is period doubling, and what is physically happening in a star to lead to period doubling?


texts: Difference in gravity on both sides of the moon

texts:
How much (if any) difference in gravity would you feel if you are on the surface of the moon and:

standing right 'under' earth (exaclty between the moon and the earth) and
on the opposite side of the moon (So both, the moon and earth are exactly under you)


texts: Do great spot like features favour appear in south hemisphere?

texts: As we know, both Jupiter and Neptune have great spot like features in South hemisphere, and Saturn has dragon storm in south hemisphere, is it true that great spot like features appear in South hemisphere more frequently? If so, what is the reason?


texts: Were radio spectroscopic observations ever used to first measure a distance successfully?

texts: Usually, especially for an extragalactic object, its redshift is determined by optical spectroscopic redshift. But the angular resolution of early radio observations is poor and an optical counterpart needs time to be found. Is there any celestial object whose distance was determined firstly by the redshift of radio spectral lines?


texts:
I have been looking at several articles on the dating methods of craters, but I am wondering how exactly the "absoluteness" creeps into it. I came across several methods that link crater "freshness" (i.e. the 7 degrees of freshness by Pohn and Offield (1970), see e.g. Google ebook) to the number of superimposed craters to rock abundance to thermal inertia etc. and these are all supposed to be an indicator of age, but what is the gauge for that? How do you convert that to a number of years?
The literature seems to call these "absolute model ages". I have seen it mentioned that you obtain them from fits to crater production functions.
So how do you obtain absolute model ages, really?


texts: How close would the sun have to get to Earth for there to be severe consequences?

texts:
According to earthsky.org the Earth gets 3 million miles closer to the Sun than its usual distance of about 93 million miles on average.

My question is, how close would the earth have to be to the sun for there to be problems with our survival?


texts: What is radiation pressure and how does it prevent a star from forming?

texts:
This is a follow up to: Is there a theoretical maximum size limit for a star?

The answer there talks about the radiation pressure preventing a star from forming.

What reaction is causing this radiation pressure and why does this potentially prevent a star from forming?


texts: Does a merging massive binary black hole ‘emits’ more than one gravitational wave?

texts: If we want to use gravitational waves (GW) to determine the Hubble constant, we need to find the source in the electromagnetic spectrum (EMS). However, we need to be lucky to ‘see’ it simultaneously in EMS and GW. This is a problem, but do BBH not ‘emit’ more than one GW? E.g. every time they spiral? Because than we have time to find it in the EMS, no? What makes it so difficult? Thanks in advance!


texts: Why are the moons of Neptune sorted by diameter?

texts:
The five moons closest to Neptune are sorted by increasing size. Naiad < Thalassa < Despina < Galatea < Larissa. The trend continues with Proteus and Triton, only interrupted by Hippocamp, which is smaller than the closer moons.
Is this ordering due to pure chance? Is there a reason why moons closer to Neptune should be smaller? Is it possibly an observational bias?


texts: Do novas create a nebula? Could a planet survive if its star exploded in a nova?

texts:
Does a nova, caused by runaway fusion in hydrogen accreted onto a white dwarf from a companion star, create a visible nebula?

If a planet were in a circumbinary orbit around a binary pair that produced a nova, would it survive?


texts: As I understand it, the difference between a reflector and a refractor is that a reflector uses mirrors on the back to reflect the light to the secondary mirror then the secondary mirror reflects the light to your eye focusing the image with each reflection. How then does a refractor use a lens to sharpen the image? Does it just sharpen the image and shoot it to the prism at the back, and into your eye?


texts:
This website shows a telescope projecting the sun onto a blackboard:  https://astronomyconnect.com/forums/articles/2-three-ways-to-safely-observe-the-sun.21/
Why isn't the board catching fire?   You can easily start a fire on a sunny day by targeting the focal point of a magnifying glass onto something flammable.  Why isn't the telescope in this picture doing the same thing?

Photo by Luis Fernández García


texts:
In different papers I've encountered time indications in BJD-2450000. I know that BJD stands for Barycentric Julian Date, but I don't understand the meaning of the suffix -2450000.

This is an example of what I'm talking about (table is taken from this paper).
I'd like to know how I can convert this in JD or just "normal" BJD if that makes sense?


texts: Did NASA/JPL get "waning" and "waxing" backwards in this video?

texts:
The NASA JPL video What's Up: September 2019 Skywatching Tips from NASA says (per both the closed captions and audio):

  We’re in a several-month period right now when the new Moon falls right around the end of each month. This means we get to enjoy lovely waning crescent moons at dusk for the first few days of each month, and delightful waxing crescents in the predawn sky near the end of each month.

Are "waning" and "waxing" used correctly here or is it just the opposite of what's said?
Would the video be correct if "waning" and "waxing" were replaced by "setting" and "rising"?


texts: Does Mars' "Northern ocean" have another name? What about the "Inland Sea"?

texts:
The March 15, 2019 Phys.org article A NASA spacecraft may have explored the edges of an early Mars sea in 1997 shows a stylized map of an area on Mars where the Mars Pathfinder landed and uses colors to indicate where bodies of surface water might have once existed.

They are labeled as

Northern ocean
Inland Sea

and I noticed they are capitalized. Do these reflect proper nouns and names of possible bodies of water that are recognized by planetary scientists or Martian cartographers (or should I have said Mars cartographers?)? Do these possible bodies of water have other names as well?


texts: Which would be safer: removing hydrogen or adding hydrogen to our sun?

texts:
If we want to extend the lifespan of our habitable zone safely, would it be safer to institute a yearly 'tax' on our suns outer layer of hydrogen, or a yearly 'subsidy' to our suns outer layer of hydrogen?

(the feasibility of said tax/subsidy is beyond the scope of this question)

Another way to ask this question:
Assuming the core of the sun remains untouched,
but its outer layer of hydrogen were reduced by half,
would the suns main sequence remain intact for a longer time?

To avoid ending up with a white dwarf and its associated bad weather, what would be the ideal point to reverse the tax and re-subsidize?


texts:
How is dark matter different from ordinary (visible) matter?

1) Does dark matter consists of invisible astronomical objects like white dwarfs, black holes?

2) Has dark matter not been directly observed?

Is there a better explanation for dark matter. I could not understand completely the difference between dark matter and ordinary matter.


texts: A book I recently purchased describes Dirac's proposed variation of Newton's G, and why that specific proposal wouldn't work because the earth would have been too hot for the known history of life. A footnote explains that the luminosity of the sun is proportional to the seventh power of G, which seems incredible to me. I know that this has to be a simplification, since stellar dynamics are annoyingly complex, but if we put that aside, is this essentially correct, or is it a typo? What would be the theoretical justification for this in broad terms? (eg, "Gravitational pressure is proportional to this exponent of G, but since that also shrinks the size of the star, which increases the pressure, it's actually this exponent of G, and the rate of fusion is proportional to the gravitational pressure squared, so you end up with 7").


texts: How can you verify the theories presented on TV shows like "How the Universe Works" is current and/or valid?

texts:
There are many TV shows such as "How the Universe Works" that explain theories about the origin of the universe such as the Big-Bang Theory.

What resources can I access to verify the scientific thinking on the origin of the universe theories presented on such TV shows?


texts: Is the location that published calendars use to calculate the seasons one specific location on the earth?

texts:
All the calendars in my house say the fall equinox is Tuesday the 23rd. All the astronomical programs and astronomical sources say it is it is Monday the 22nd in Ohio. Since the time given is 10:29 p.m. EDT all of the US starts fall on Monday. I’m guessing the calendars use Greenwich Time. This same discussion also applies to when calendars show the full moon dates.

I know this isn't exactly an astonomy question, but I thought this site was the closest I could get.


texts: Longest time after an image was taken when a new solar system body was discovered from it?

texts:
This answer to If there's nothing special about Pluto, why was it discovered so early? says:

  In fact, often objects are discovered to be in pictures from long ago, such as Orcus, discovered in 2004, but pictures were taken of it in 1951.

This led me to wonder about solar system objects that were in fact discovered in very old data.

Answers mentioning notable discoveries that are old but may not necessarily be the "oldest" are also are welcome!


texts: Do apparent magnitude (V) and photometric magnitude (specifically in optical V band between 500-600 nm) have a similar scale to stellar magnitude?

texts:
I'm graphing a table and I used the magnitude values from this database
https://vizier.u-strasbg.fr/viz-bin/VizieR-4
(It's the V/19/data catalog for "Masses and Ages of stars in 68 open clusters", Piskunov 1980 in VizieR)

I wanted to know if this scale(V mag)  works in the same way as a stellar magnitude scale where the fainter objects have a higher magnitude number. Generally speaking for a star's magnitude number does a larger value automatically imply less intensity?
Also if anyone could refer to any databases that contain stellar magnitude values for different open galactic clusters that would be so helpful.
Thanks!


texts: Is there any free stacking program that stack images which are not aligned?

texts:
I liked to photo deep-sky-objects and planets with my camera Canon sx130. But I have never stacked images because they were too much apart. I was looking for some free astronomy stacking program for stacking photos that aren't perfectly aligned, like doing the mosaics, but I found just Deep Sky Stacker. It does not stack images that aren't aligned and I have some troubles with it.

For examples, I want to stack this image:

¸¸¸¸¸¸¸¸¸¸
¸¸¸¸¸¸¸¸×¸
¸¸¸¸¸¸¸¸¸¸
¸¸¸¸¸¸¸¸¸¸

and this image:

¸¸¸¸¸¸¸¸¸¸
¸¸¸¸¸¸¸¸¸¸
¸¸¸¸¸¸¸¸¸¸
¸×¸¸¸¸¸¸¸¸
¸¸¸¸¸¸¸¸¸¸

("¸" is the dark sky and "×" is a planet.)

Any help?


texts: Extract anomalies from two-line element set (TLE) on ISS (International Space Station)

texts:
I am faced to a problem of interpretation illustrated on figure below :

From wikipedia : Two-line element definition, I can extract the
following meaning anomaly  : 308.7017 ° (in degrees)

But in the 2 questions, I am asked to find :

1) at which anomaly is the ISS at the two lines epcoh

2) at wchich anomaly is the ISS on December 1st 2018.

I think the answer to question 1) is maybe what I have extracted above (308.7017 °)

But how to compute the anomaly of ISS on December 1st 2018 ?

I don't undertand very well the difference between these two anomalies requested.

If someone could help me, this would be nice.

Regards

UPDATE 1: Sorry, I must precise that I talk about mean anomaly.


texts:
Just a little something whilst a dark matter pioneer moves onto a better place.

This commentator writes:

  My understanding is that dark matter was "discovered" when trying to find out why stars in the outer edge of a galaxy rotates with the same speed as the inner. A ten-fold increase in number of galaxies doesn't seem to affect this.

When reading the history - this simple explanation doesn't quite come across. Perhaps the statement above is oversimplified. For example:

mass in the galactic plane must be greater than what was observed (doesn't quite talk about comparative rotational velocity)
estimated its mass based on the motions of galaxies near its edge (doesn't quite talk about comparative rotational velocity)
some unseen matter provided the mass and associated gravitation attraction to hold the cluster together (doesn't quite talk about comparative rotational velocity)
measurements of galaxy rotation curves (what is a galaxy rotational curve? Shouldn't it be a straight line if they're rotating at the same angular velocity?)
gravitational lensing of background objects by galaxy clusters (not talking about comparative rotational velocity?)

My question is: Is it true dark matter was discovered whilst looking at the rotational speed of galaxies?


texts: If Earth didn't rotate, would we feel heavier?

texts:
Suppose the Earth's rotation slowed for some reason. Would the lack of centrifugal force cause us to feel heavier than normal?

Likewise, if Earth's rotation increased, would we feel lighter as centrifugal force lifts us from the ground?


texts: How are the three character IAU Minor Planet Center's Observatory Codes assigned? Why are some letters so popular?

texts:
Answers to Latitude, longitude and altitude of Ckoirama Observatory; where can things like this be looked-up? point to https://www.minorplanetcenter.net/iau/lists/ObsCodesF.html which currently lists 2198 observatories.

The first character of the three character codes has 36 possible alphanumeric values. and the second two are only digits. That allows for 36*10*10 = 3600 possible codes.

I plotted the ones that are currently used and the pattern is interesting. Why are some letter codes fully used (all 100 two digit suffixes are populated) whereas others are completely empty? I haven't noticed an alphabetical associations.

Y = 0 to 9 are the digits and 10 to 36 are A through Z: Shading means the code is used.

For some reason I can't stop comparing the data to the famous Arecibo message, a tiny 1-bit bitmapped image beamed to space containing DNA, amino acids, our solar system and other goodies:

import numpy as np
import matplotlib.pyplot as plt
# blob is the unformatted lines from https://www.minorplanetcenter.net/iau/lists/ObsCodes.html
threes = [line[:3] for line in blob.splitlines()]
key = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'
a = [[x] for x in key]
for t in threes:
    a[key.find(t[0])].append(t)
b = np.zeros((36, 100))
for i, thing in enumerate(a):
    c = [int(x[1:]) for x in thing[1:]]
    for d in c:
        b[i, d] = 1
plt.imshow(-b, interpolation='nearest', cmap='gray')
plt.gca().set_aspect(2)
plt.show()
pairs = []
todegs = 180/np.pi
for line in blob.splitlines():
    try:
        cos, sin = [float(x) for x in (line[13:21], line[21:30])]
        lat = todegs * np.arctan2(sin, cos)
        lon = float(line[4:13])
        pairs.append([lon, lat])
    except:
        pass
print(len(pairs), len(blob.splitlines()), float(len(pairs))/len(blob.splitlines()) )
lon, lat = np.array(list(zip(*pairs)))
lon = np.mod(lon+180, 360) - 180
plt.figure()
plt.plot(lon, lat, 'ok', markersize=1)
plt.ylim(-90, 90)
plt.xlim(-180, 180)
plt.gca().set_aspect('equal')
plt.show()


texts: If the Earth rotates on its own axis, then what do we mean by sunrise and sunset?


texts: Recently I read about comet C/2014 Q2 (Lovejoy).The thing got me amazed was that scientists found it was dumping ethyl alcohol all along its way, at a rate of around 300-500 bottles per second. So I want to ask how common is alcohol in outer space, and how did it form? I usually imagine space as an inert mixture of dry dust and light gases but it turns out there are sometimes alcoholic compounds.


texts: Stellar temperature ~ frequency relationship?

texts:
According to Wikipedia, 76% of all stars are of spectral type M, 12% are of K, 7.5% are of G, etc. (see this section on Stellar Classification). This is very helpful, but is too vague, as the temperature span is too large and irregular to derive an accurate distribution formula for the temperature frequency of a zero-age main sequence star.
Is there such known distribution histogram for zero-age main sequence stars, plotted as $x=T_{eff}$ and $y=\text{percentage of stars}$?


texts: How does a gas giant planet form like Saturn and Jupiter, and why just gas? Why didn't they form any solid surface?


texts: What should be the declination of a star to be marginally circumpolar given the latitude & height from ground?

texts:
If I'm given the latitude, I can easily find the declination by using
declination = 90 - latitude.
I can also find the angle of dip from the height but should I add or subtract it from the declination I found?
Also I have to consider atmospheric refraction but I don't know how.


texts: How to get the solar zenith from the longitude and latitude?

texts:
I have real data of “Direct Normal Irradiation” and “Diffuse Horizontal Irradiation” and I should calculate “Global Horizontal Irradiation”. I have also longitude, latitude and elevation.
Could you advise me how I can get zenith from these data, then to calculate the GHI?
Many thanks,


texts:
I'm trying to compile a table showing the wavelengths and flux at m=0 for WISE, Spitzer and GALEX, but I can't find the right data for GALEX. Here's what I have so far:

        Band    Wavelength      F_0
                micrometers    Jansky
        ------------------------------
        FUV     0.152
        NUV     0.227
        U       0.365           1810
        B       0.445           4260
        V       0.551           3640
        R       0.658           3080
        I       0.806           2550
        J       1.220           1600
        H       1.630           1080
        K       2.190           670
        W1      3.4             309.54
        W2      4.6             171.787
        SPIT8   8               64.13
        W3      12              37.674
        W4      22              8.363

I got most of the info from here. The information I found for GALEX on page 687 of this paper gives m_0 for NUV and FUV, but no F_0. How can I change m_0 to F_0?


texts:
After SOHO began watching the Sun from space with its coronagraphs 24/7 it was discovered that there are comets passing very close to the Sun quite regularly, and while some are destroyed, some are seen to emerge and continue on.
From these observations and from all other comet and asteroid observations there should be quite a collection of objects whose orbits take them very close to the Sun.
Is there any way to find out What is the closest an asteroid or comet has passed the Sun and survived?

Here is a GIF I made for this answer also used in this answer. In addition to some comets you can see Venus on its approach to occultation by the Sun in 2016. Seeing the Pleiades so close to the Sun is exciting as well!

These LASCO C3 images from SOHO were downloaded sohodata.nascom.nasa.gov/cgi-bin/data_query. The square frame is about 15.9 degrees wide.


texts: How is the density parameter estimated via cosmic microwave background anisotropies?

texts:
In 2003 and again in 2018, the density parameter was estimated based on measurements of the cosmic microwave background anisotropies. Can someone please explain how that works? Why do the anisotropies reveal something about the universe’s current density?
I don't have a background in physics or astronomy.


texts: Does science need support from religion or philosophy to explain the creation?

texts: Science is trying to explain how the universe is made. The beginning of the universe explains the Big Bang theory. But what happened before Big Bang? Science cannot go before that. So, does this mean that always there be space for religion and philosophy to explain things that science cannot?


texts:
What are criteria to compare and evaluate different 12 inch truss Dobsonian telescopes? I'm primarily interested in comparing mirror quality and weight (price I can do myself of course). Two examples of the category I'm asking about are the Sky-watcher 300P Flextube and Meade Lightbridge.

One of them appears to have a fan to cool the mirror, how can I evaluate that in terms of a positive or negative towards performance?


texts: how steady over time are right ascension and declination values?

texts:
I understand that astronomers use right ascension and declination for the position of stars. However both these values strike me as very anthropocentric and, more importantly, unreliable over time as I understand that the Earth wobbles a bit over time so the celestial equator shifts and I guess Sun's rotation around the galaxy and Milky Way's own trajectory should play a role in depreciating the accuracy of those values over time.

So my questions are:

[1] how steady are right ascension and declination values for distant stars over time?

[2] is there a more stable coordinate system or do we lack an absolutely fixed frame of reference?


texts: How do I calculate the hour angle of a planet's moon from a point on that planet's surface?

texts:
I'm worldbuilding and I'm trying to construct a standing stone calendar that'll accurately denote where to look for my planet's moon at its four major phases from 53.8 degrees South. So, I need the h0, or hour angle at moonrise and moonset, for my moon at each of these four points in its orbit from this point on the surface of my planet.

The problem is, I can't even find any equations for how to find these numbers for Earth's moon, let alone another planet's moon.

So, here goes...

My planet, Jasmi, has an axial tilt of 14.92 degrees and an orbital period about its star of 640.04 Earth days.

Jasča, my moon, has a semimajor axis of 373168 km and an eccentricity of about 0.01.

Its orbit is 5 degrees inclined off the planet Jasmi's equatorial plane, toward the ecliptic.

The longitude of the ascending node is 358.11 degrees.

Jasča's argument of periapsis is 9.23 degrees.

Now, since this is not supposed to be Earth, I could have measured the longitude of the ascending node from any old direction, but I'm already at my wits' end with trying to calculate everything properly and I want the night sky to be at least somewhat familiar to readers, so I just plopped our own Earth's celestial sphere onto Jasmi and offset its ecliptic to be 8.58 degrees closer to the celestial equator to account for the difference in planetary obliquity. So we're still setting longitude of the ascending node in degrees east of the First Point of Aries.

Any guidance y'all could offer would be very very much appreciated!

Thanks,

<3 R


texts: Can the Sun be used as a gravitational lens to achieve better telescopic viewing? Can this effect be practically used to view celestial objects?


texts: Things to be aware of when designing a custom equatorial fork mount

texts:
I'm still relatively new to amateur astronomy, and I would like to get into astrophotography. I've looked at different equatorial mounts, and I think I like the benefits of fork mounts. I'd like to design one that can be adjusted to fit a few different scopes and probably a camera with large lens. The way I would do this is by making various adapters, and possibly allowing the fork width to be changed.

As I have never done this before, what issues should I expect to encounter, especially with regard to tracking accuracy?


texts: We call dark matter dark matter, because of the way it acts on galaxies, but why do we look at dark energy as an energy? With $E = m c^2$, can't dark energy be mass as well? Why dont we look at dark energy as a mass, in the same way as we look at dark matter as being a missing mass (as halo around galaxies)?


texts:
I am not sure if this is the right place to post at, but I tried to find the best place to do so.

To my point, I wonder if it is posssible, to determine where north is, just by one shadow, that means no Sun, no time, just a shadow in a picture or video...

Do you get my point? Like this picture, can I tell where north is?

Like this image:


texts: How to communicate Fine Structure Constant to aliens?

texts:
Sorry if this is not the right question for this channel, I just need to know and there is no good place to ask.
It is said the Fine Structure Constant α is one of the most fundamental constants in nature. Unlike the likes of the speed of light, α is dimensionless. I have heard an argument that when we encounter intelligent aliens sending the α at them is the best way to let them know how much we know. It was said that we should of course send the constant in the binary numeral system.
My question is: how do you communicate a fraction (α = roughly 1/137) in binary so that even aliens can decode it?


texts: Spotting the booster rocket of the Chinese space station?

texts:
SpaceWeather.com's newsletter states

Sky watchers are reporting two new objects in the night sky: China's new space station and an out-of-control booster rocket that helped launch it. The tumbling booster is flashing brighter than some of the brightest stars in the sky.

My usual star-gazing app does not have the rocket booster in its database (yet), and I did not manage to spot it yet (from cloudy central Europe). Is there an official name for the object? Could somebody help me how to find it in the remaining 2 days while it is still in orbit around Earth? Or maybe somebody could even share a picture or two?
Bonus question: Maybe somebody could also enlighten me in how far any single-use booster-rocket could be in-control after it ejected its payload? Wasn't that the default for many years? Also: Now that we can observe the rocket we know the trajectory and could also forecast when it will crash and where, aren't we?
References

U.S. Space Command tracks Chinese rocket for uncontrolled re-entry from orbit


texts: Near Earth Objects (NEO's) have been mapped by NASA and JPL since 1990's. The graph below shows how many of these potential planet-killers have been found and classified over the years. I am curious as to why there have been so many more 140 meter to 1 km NEO's discovered, as opposed to NEO's with a diameter of 1 km or more. I think it is as simple as there are just more "smaller" objects left over from the formation of the solar system? Am I correct in saying this, or are there deeper astronomical processes at play here?


texts: Webcam set up small circle tunnel vision type view

texts:
I've just adapted a webcam for my telescope.

I'm new to astronomy so I'm not sure if the view I get is right.

When I attached the webcam to my Barlow lens, the view on my laptop becomes a tiny circle of light, like a tunneling effect.

Is this normal? How could I make the view I see on screen lager?

Here is what I'm currently seeing:


texts:
Is there a limit to how massive a planet's ring can be?

If there is, how does it depend on the mass of the planet, presence/absence of moons, distance to the star or other parameters?


texts: Question on Equatorial and Galactic coordinate systems

texts:
I'm currently working with proper motions and have needed to convert my galactic longitude and latitude into right ascension and declination (ie from galactic to equatorial coordinates). This is fine since NED have a tool to do this, however once I convert into Cartesian coordinates I do not get the same results as I had for the galactic-cartesian conversion.

I understand that the conversion from galactic to cartesian is

x = Dcos(l)cos(b)

y = Dcos(b)sin(l)

z = Dsin(b)

where D is the distance to the galaxy in question in my case.

Also for the equatorial to cartesian transformation

x = Dcos(RA)cos(DEC)

y = Dcos(DEC)sin(RA)

z = Dsin(DEC)

I have done this for both my galactic coordinates and converted equatorial coordinates, but I do not get the same x,y and z. Should this be happening?


texts: Is it possible that Titan is a kuiper object captured by Saturn?

texts:
I think Titan may be just captured by Saturn, not formed with Saturn together, because:

It has so much nitrogen and methane, which is similar to Pluto and Triton
Except Titan, other moons of Saturn are very small, which other moons may be ejected by Titan during capturing process, similar to Triton and Neptune
According to Nice model, Neptune swapped position with Uranus, while Saturn pulled Jupiter away from the Sun, so I think the orbit of Neptune and Saturn is quite close at past, so if Neptune can capture kuiper object, Saturn also can

Is it possible that Titan was a Kuiper object?


texts: What is the history of the average pressure, density, and temperature of the matter in the universe over time?

texts:
This question is inspired by this more specific question where Cerelic wanted to know if conditions were suitable for liquid water to exist during an epoch when the characteristic temperature of the cosmic black body radiation was near room temperature. The answer is No, because the pressure was too low by then, but that raises the question of what the temperature of the medium was at a time when the pressure was near Earth sea-level pressure.
A log-log graph of temperature, pressure, and density of baryonic matter starting at the Big Bang would be quite interesting, especially if it were annotated with familiar references. It might need to have a special scale during the inflation era. Does such a thing exist in the literature?


texts: How big can a black hole become and  how small can a black hole become?(minimum and maximum dimensions of a black hole)


texts: Is a Procyon - brown dwarf - Jupiter - superearth - Mars - Pluto system theoretically possible?

texts:
I once conducted a thought experiment and with some amateurish calculations based around Hill sphere and Roche limit came up with semi-major axis distances like this:

Procyon-BD:  3.6 AU
BD-Jupiter:  0.7 AU
Jupiter-SE:  0.12 AU
SE-Mars:     0.02 AU
Mars-Pluto:  0.0033 AU

Assuming respective masses of: 496 260 earths, 15 000 earths, 313 earths, 6 earths, 0.107 earths and 0.00218 earths.

Clarification: I use names of specific bodies (Procyon, Jupiter etc) but a Procyon-like star, a Jupiter-like gas giant and so on would be more precise. Think of them only as examples of bodies with such masses.

Could such a system be possible?


texts: In my mid-forties, I believe I have seen Pluto for the first time with the naked eye

texts:
Early this morning going out on the balcony, I looked up on a star chart app to verify it was Jupiter I was seeing.  Then I noticed the alignment of Mars, Saturn, and Pluto on the app.

Never being able to identify it before, I stared at where Pluto should be and I'm pretty sure I saw it.

My only question is - since it's said that planets shine and stars twinkle, it did seem that Pluto was flicking a bit.  Is this normal?  Something to do with the relatively low luminosity and greater length of space?


texts: Is there a limit to how fast spacetime can bend/warp around an accelerating object?

texts:
If in the future a spaceship can accelerate to nearly the speed of light, will it encounter any kind of resistance/drag from the spacetime in front of it not being able to bend/warp fast enough around it?

It is my understanding that as an object accelerates, its mass will increase and this increase in mass should result in the object having more gravity, and this gravity will continue to increase as the object's speed increases. The creation of more and more gravity should result in an increased area of spacetime surrounding the object being bent/warped.

So, if there is a limit to how fast spacetime can bend/warp around an accelerating object, along with an increased area of spacetime being affected by an accelerating object's increased gravity, will this spaceship start to heat up and begin to melt the faster it accelerates?


texts: How is the heat production inside moons compliant with Conservation of Energy?

texts: A Nova episode said that during stages of some moons' elliptical orbits, the moon is stretched, and friction causes the inside of the moon to heat up. I do not understand how this is compliant with the Law of Conservation of Energy - what energy would be lost to make the inside of the moon heat up?


texts:
On July 13, 2018, just after the clouds had lost their color, a bright object became visible about 20 degrees above the western horizon (there was a hill).
It moved slowly east and had colors similar to what the clouds had recently.
In my spotting scope it was circular, sort of looked like the ring nebula, with an apparent diameter bigger than Jupiter.
It wasn't a satellite, nor an airplane. Weather balloon? Any ideas?
I know this isn't exactly an astronomy question, but I couldn't find another forum interested in such phenomina.


texts:
So I'm currently working around a specific variable star, α Ori and I want to determine it's luminosity...I do only have the light curve (driven from V-optical photometric band - Johnson V from AAVSO)... Is it allowed to utilize it to compute for Luminosity L? The distance modulus doesn't seem to fit since it is for 'visual' magnitude (Using our eyes) so I guess it requires a different approach?
I tried to check in SDSS tho for the other photometric system...But since it avoids the galactic plane and α Ori is "bright" enough, I'm sure the detectors will be saturated so I opted it.
Since I'm new to this kind of stuff, I'm looking forward for your help/s, suggestions and others. Also, I may have missed out something so I apologize.
Best regards,
CGHA


texts:
What is the relation between the wavelength and the radial velocity? I have a data: wavelength and intensity and I would like to plot intensity with respect to radial velocity - similarly as in the picture (source)

EDIT

I have a file of alpha H with 2 columns - wavelength (in angstrom) and intensity (normalized for 1) and I would like to plot intensity with respect to radial velocity. I use in gnuplot:

c = 299792458
lambda_0 = 6562.817*10**(-10)
plot 'file.asc' using (c*($1*10**(-10)-lambda_0)/lambda_0):2

Why I get the wrong range on the x-axis?


texts: Can two planets in an empty universe meet/be pulled together?

texts:
For this question assume that the entire universe is completely empty. The universe is not expanding or contracting, it is completely motionless since time immemorial and has never moved before.

Only two identical earths without moon are left in this entire universe and they are 5 billion light years apart. From their starting position the earths do not spin or move at all they are completely motionless.

Would these planets ever meet or at least be pulled together?  Or are the their respective gravities too small to have any meaningful effect?


texts: Naked-eye limiting magnitude: mismatch between Wikipedia, online converter and Shaefer's formula

texts:
I'm trying to make sense of the difference between the reference Sky Quality Meter (SQM) readings given in the Wikipedia article on Bortle scale, online converters like [1] that are based on Shaefer's paper [2], and an equation from [2] itself.
Wikipedia article gives naked-eye limiting magnitude (NELM), as well as corresponding SQM readings for each Bortle class. The original paper [3] by Bortle only specifies NELM, so the SQM column in Wikipedia must have been derived from the NELM values.
But in the very first value, SQM reading for Bortle class 1, the Wikipedia article fails to match what the online converter yields (other such converters use the same formula). Namely, the "excellent dark-sky site" NELM is 7.6–8.0, for which Wikipedia gives lower bound of SQM reading $21.99\,\mathrm{mag/arcsec}^2$, while the converter [1] yields $25.50\,\mathrm{mag/arcsec}^2$. The upper bound, NELM=8.0, is converted to $22\,\mathrm{mag/arcsec}^2$ in Wikipedia, while the online converter simply fails, yielding NaN. This failure is because the formula used by the converter has 7.93 as the maximum possible NELM.
If we use equation $(18)$ from [2] instead (with $k_v=0.3$ and $F_s=1$, as suggested in the text under the equation), we'll get the range of SQM readings $24.35\,\mathrm{mag/arcsec}^2$ to $26.32\,\mathrm{mag/arcsec}^2$, which doesn't match Wikipedia either.
Also, these values $\ge22\,\mathrm{mag/arcsec}^2$ seem to also conflict with [4]:

The natural level is around $21.6\,\mathrm{mag/sec}^2$ at a solar activity minimum.

Besides, an answer on Astronomy.SE implies that values of NELM larger than 6 don't make any sense at all.
So, how to make sense of the conflicting results? Which of these are right, which are wrong? Or do they correctly describe different things?
References

K. Fisher. "Conversion Calculator - NELM (V) to MPSAS (B) systems Rev. 8/2006"

Schaefer, B. E.. "Telescopic limiting magnitudes". Astronomical Society of the Pacific, Publications (ISSN 0004-6280), vol. 102, Feb. 1990, p. 212-229.

Bortle, John E. (February 2001). "Gauging Light Pollution: The Bortle Dark-Sky Scale". Sky & Telescope. Sky Publishing Corporation.

Dark Skies Awareness: "Sky Brightness Nomogram"


texts: Calculating Postition of ISS / Hubble in space

texts: How do I calculate the positions of above satellite in space ?


texts: What effect does the Earth's rotation have on plate tectonics?

texts:
I've been reading up on plate tectonics and found that there are several different driving forces involved. One of the main categories of driving force mentioned on Wikipedia is forces related to the Earth's rotation. The section on driving forces related to Earth rotation presents the information as a sort of historical dialogue of the scientific process, which is somewhat helpful but has left me confused as to exactly what role the Earth's rotation does play in plate tectonics.

So how does rotation affect the movement of plates?


texts: Do retrograde spin-orbit resonances exist?

texts:
The end state of rotation of an initially fast-spinning prograde terrestrial planet (in the absence of additional forcings such as "thermal tides" in an atmosphere, e.g. Venus) is a spin-orbit resonance of the form $n + 1 : 2,\ n \in \{1,2,3,...\}$. Examples being the common 1:1 resonance ($n=1$), and the 3:2 resonance of Mercury ($n=2$). See also this paper (Correia & Laskar, 2001) discussing the probability of Mercury's capture into higher-order resonances, e.g. 2:1 or 5:2.
What about the case for a planet with retrograde rotation? Do there exist retrograde spin-orbit resonances, e.g. extending the sequence to negative $n$, giving resonances such as $-1:1$ or $-3:2$, and would there be any probability of capture into such a state?


texts: What is the physics of a gas mass subject to gravity in space?

texts:
There are many formulas for atmospheric pressure on earth, but how does gas behave in free space?

I am thinking about why stars form. I am guessing that the gas density will influence pressure, as well as gravitational force. So both factors must determine if a given gas mass in a given volume in space will collapse into a star.

So a more well defined question would be: what is the relation between mass and volume in space (let's think about hydrogen only) so that this mass will collapse into a star (or at least forms a spherical body)?

A second question is: given a mass and volume so that the gas collapses into a spherical body, what is the pressure vs. distance from the center of the sphere?

Quite interesting I think!

Any ideas?


texts: Why are there no terrestrial planets with a subsurface ocean?

texts: Subsurface oceans in satellites are pretty common: Europa, Enceladus, Ganymede, Callisto, maybe Pluto... This is due to tidal heating of their host planet, Jupiter and Saturn, which heats up the inner ice of those satellites. However, planets don't exhibit this inner ice layer, so they don't usually have subsurface oceans (except Pluto or Ceres, if you can call them "planets"). Why is that? Only small bodies like satellites present this inner layer of ice? Is there any Earth-type exoplanets that exhibit this inner layer of ice that could potentially melt down to liquid water? And if there is, why some cold planets have inner ice layers and others don't?


texts: Whats the farthest distance a spacecraft has been detected (except by radio)

texts:
As far as I understand it, one can not hide spacecraft, because the heat from the engine would be very visible.

Now I tried searching for the farthest distance an actual spacecraft has been detected besides its radio signals that are intentionally sent to us. Unfortunately I could not find any data on this.


texts: What was earth's mass before the collision with theia?

texts:
Assume the "Big Splash" or "Giant Impact" (whichever name you prefer) theory is more or less correct.

EDIT: With so many variations of this theory, with so many names and qualifiers on them, such as "direct impact theory" and "glancing blow hypothesis", etc., let's assume that the impact was direct enough that nothing of Theia (other than Earth and the Moon) was left that was large enough to for its gravity to compress it into a hydrostatically equilibrious shape, and scatters too much to coalesce (on its own) in to anything large enough either.

Assume "Theia" was the size of Mars (some variations on the theory use other sizes, but for the sake of this question, let' stick with Mars)

How big was Earth before the impact?

It can't be as simple as "Earth+moon-Theia(mars)=pre-impact size", can it?  Because there must have been some debris ejected completely from the earth+moon system, right? or was it such a small amount that it can be considered negligible?


texts: What was that Asteroid/Comet in the 90s?

texts: I am now in my mid 20s and I remember when I was 6-10 years old, there was a comet or asteroid that flew really close to earth. I remember being able to see the object very clearly as if it were the moon. I also live in the west coast of the United States. What was this object?


texts: What's the largest non-spherical astronomical object in the universe?

texts:
Some asteroids and comets are non-spherical. But is the nature of big things and gravity so that large things in the universe are always spherical? What is the biggest astronomical object in terms of volume out there that's not spherical?

Note: Astronomical object is a defined term. See Astronomical object - Wikipedia.


texts:
Do we really know that the origin of gold was in a supernova explosion?

I am talking about the gold we find here on Earth.

So then how are we sure that Gold actually came from these explosions.  Did they go back some million years ago and figure out that Gold was made in supernova.


texts: Will Earth at some point stop rotating anti-clockwise, stay still, and then rotate clockwise?


texts: How do I get started in astronomy data analysis?

texts:
I am learning some machine learning (I have previous background in statistics) on my own, and I am interested in getting some hands-on practice by doing some ML analysis in R and Python.

Given how mad I am about astronomy, I would love to get my hands dirty on doing some ML using open data. I am aware that organisations like NASA have lots of open datasets. What I am less clear about is the kind of analysis that needs to be done.

So, I'd like to know if there is any website where I can get a repository of space data (images, signals, whatever) and which asks what analysis is required; kind of like Kaggle where one gets the dataset and has the required question which one can attempt.


texts:
if we have spherical mass held by its self-gravity, there will be a limit to distance - let's say it is $r$ - that the center of this body may approach a much more massive body (lets we call $M$ body).

Our spherical mass here would be held - as I understand - by gravitational tidal force. But the question is what is the distance $r$ between to masses his effect will hold?

$$r=\left(\frac{2M}{m} \right)^{1/3} \times R$$

$m$ - Spherical body mass

$R$ - Spherical body radius

$M$ - Massive body

I guess this is the equation for this limit, but I don't understand the conditions of this limitation.


texts: Brown Dwarf transcending past the sun with the naked eye

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I seen with my naked eye in mid 2014 from Ca. right after sunrise a black spot in front of the Sun.I observed it for about a hour with my naked eye. Here is a picture that closely resembles what I observed, except the object was a third the size of the black dot in the posted picture and not in the middle of the sun but off to a angle.Since Venus did not transcend with the Sun in 2014 my speculation would have to say it might have been a Brown Dwarf.The link provided below of civilizations from the past documenting Venus transcending with the Sun throughout history could be observed with the naked eye,So a Brown Dwarf would be easier to see with the naked eye.The picture is from google and is the closest image I can give to what I seen.I have no knowledge of the picture and can not determine if it is real or fake.

Proof of venus or a sunspot seen with naked eye transending past the sun
http://sunearthday.nasa.gov/2012/articles/ttt_73.php